Under normal operating conditions load connections from a step-down distribution transformer supply electrical power to loads in a variety of settings such as residences, businesses, and farms. The loads are mainly machinery, lighting and other electric devices. The load circuits supplying power to the loads include both load conductors and cooperating neutral conductors. The neutral conductor is, like the load conductors, connected to the transformer but unlike the load conductors, it is also connected to a ground connection of the primary power source such as a high voltage power line.
In addition to grounded neutrals for completing load circuits, many of the loads are themselves grounded. Additional load grounding include connections to grounding electrodes, such as conductive rods driven into the earth, well casings, and water pipes feeding houses, barns or other water supply users.
In an ideal situation, load conductors supply current to an electrical "load," with the neutral conductor completing the circuit conveying the current back to the distribution transformer and its primary ground connection. But where some of the current flows through the ground connections at the loads, the current in the neutral conductor is reduced to a lesser value. The two main consequences of this occurrence are excessive magnetic fields surrounding the load and neutral conductors as well as the occurrence of stray voltage. The present invention substantially prevents current from flowing through neutral ground connections to obviate these problems.
Current will flow through the neutral grounding connections when the impedance of the neutral grounding connections drops below that of the neutral conductor's return path to power source. The voltages present upon the neutral grounding connections are said to be "stray", as they are not flowing through their intended conductors. Neutral grounding connections are desirable for safety reasons, as when a fault or short occurs in the circuit, but they are not necessary for the actual function of an electrical machine/load. Stray voltages are a known problem when humans, animals, or sensitive machinery become part of the ground circuit.
The classic case of stray voltage occurs when, for instance, a machine frame is locally grounded to the water lines of a dairy parlor. As metallic water lines offer very low resistance to current, it is probable that, should a fault occur somewhere in the grounded machine, a current will flow into the water lines. When a cow comes into contact with the piping of the dairy parlor through a water cup, or through a milking machine, some of the current may travel through the cow to true ground, i.e. the earth. This occurs where the neutral grounding connections are not connected to true ground or where the grounding is defective. This is especially likely to be a problem in areas, like barns on dairy farms, where there may be a lot of moisture.
Even where all of the loads in a particular location are properly grounded, stray voltage can occur. In this situation, stray voltage is generally from a common grounding point such as a common water pipe that is shared among a number of locations, for instance a number of power service installations in a localized area.
Past attempts at remedying stray voltage problems have included simply maintaining existing ground connectors providing common grounds for all electrical loads, providing an equi-potential plane grid underneath the flooring supporting the electrical devices connected to loads, or connecting a saturation reactor in line with one or more of the neutral conductors of the power distribution or electrical load system. U.S. Pat. No. 4,573,098 granted to Dale B. Williston on Feb. 25, 1986 teaches this latter method.
Yet another method for dealing with stray voltage was discussed in the publication entitled Net Current Control authored by David W. Fugate of Electric Research and Management, Inc., published Sep. 28-30, 1994, at the EPRI Magnetic Field Management Seminar in Lenox, Mass. This method is further described and claimed in U.S. Pat. No. 5,536,978 granted to Cooper, et al., on Jul. 16, 1996. The method and apparatus disclosed in the Cooper, et al., patent appears to parallel a method for reducing interference in telephone lines which utilize so-called "booster transformers." The use of "booster transformers" is described in section 873-500-100, Issue One, March, 1976, of Principles of Prevention and Mitigation of Inductive Interference published by Bell System Practices, AT&T.
A further related disclosure may be found in the Hertz, et al., U.S. Pat. No. 4,816,956 that defines a stray voltage apparatus having a ferromagnetic core made of a material with a particularly high initial permeability in excess of 50,000. The apparatus is located on the load side of the step-down distribution transformer.